// @if @else
@mixin blockOrHidden($boolean:true) {
    @if $boolean {
        @debug "$boolean is #{$boolean}";
        display: block;
    }

    @else {
        @debug "$boolean is #{$boolean}";
        display: none;
    }
}

.block {
    @include blockOrHidden;
}

.hidden {
    @include blockOrHidden(false);
}